This invention relates to solid state electrolytic cells and to oxygen sensors utilizing them. It has particular utility as a highly stable, rapid response lambda oxygen sensor in an automotive exhaust system.
Solid state electrolytic cells are well known. A particularly useful cell includes a solid electrolyte which selectively transmits oxygen and which includes catalytic electrodes on opposed sides of the solid electrolyte. Such cells are widely used as automotive lambda (stoichiometric) exhaust gas sensors, where they produce a voltage signal which is highly dependent on the amount of oxygen in the exhaust gas stream. It will be understood, however, that the usefulness of the invention is not limited to such sensors. For example, multiple such cells can be connected as non-stoichiometric, pumping oxygen sensors. See, for example, Kondo et al., U.S. Pat. No. 5,480,535. In other uses, when connected as a current generator, such cells act as fuel cells, and when an external voltage is applied, they can act as oxygen generators which produce exceptionally pure oxygen.
A common configuration of an automotive lambda exhaust gas sensor is a small thimble-shaped body of compacted zirconia (zirconium dioxide) stabilized with about 2-10 mole percent yttria (Y.sub.2 O.sub.3) and, optionally, 0-20 mole percent alumina (Al.sub.2 O.sub.3). The catalytic electrodes can be painted on as a platinum ink. Commonly, the outer electrode is formed by vacuum sputtering a thin film onto substantially the entire outer surface of the thimble. The sputtering process is expensive and inefficient, the electrodes are of varying thickness from one axial end of the thimble to the other, and the resulting sensors are unpredictable and have high reject rates.
The basic operation and known problems of an automotive lambda exhaust gas sensor are described, for example, in Topp et al., U.S. Pat. No. 3,978,006, Burgett et al., U.S. Pat. No. 3,844,920, Romine et al., U.S. Pat. No. 4,186,071, and Berg et al., U.S. Pat. No. 4,253,934. As set out in these patents, it is desirable for the sensor to have switching times on the order of under 200 milliseconds when the air-to-fuel ratio fed to the engine switches from lean to rich or rich to lean with respect to the stoichiometric ratio. It is also desirable for the sensor to produce smooth switches of at least about 200 to 300 millivolts when the air-fuel ratio switches. In recent years, the time required for an oxygen sensor to reach its operating temperature has also been recognized as a significant problem, and heated oxygen sensors have become standard. It is thus also desirable to produce an oxygen sensor which is well suited to introduction of a heater into the sensor structure. The background of heated oxygen sensors is well set out, for example, in Ker et al., U.S. Pat. No. 4,824,550.